This invention pertains to a method of testing a panel assembly of a color cathode-ray tube prior to sealing a faceplate panel of the assembly to a funnel section of the tube in order to detect defects in the assembly.
In manufacturing cathode-ray tubes for use in color television, three major components are assembled into the finished color kinescope or cathode-ray tube. These components are the electron gun assembly, the funnel section, and the panel assembly which includes an apertured shadow mask mounted therein adjacent to a luminescent screen of red, green and blue phosphors. Defects in any of these components can cause failure of the finished cathode-ray tube. If the defects are detected before the final assembly, the cost of scrapping the component is relatively low. However, if the defects are detected at final inspection of the tube, the cost of scrapping the finished product is significantly higher.
Due to process and material variations, shadow masks are not interchangeable. Hence, early during the fabrication of the panel assembly, each shadow mask is uniquely paired to a particular faceplate panel, so that the mask and panel then go through subsequent fabrication steps together and are assembled together in the final product. Phosphor slurries and a black matrix application are deposited on the inner surface of the panel in chemical photoresist processes which utilize the shadow mask as a photolithographic mask during light exposure steps. The light exposure steps are carried out on a photoexposure apparatus known in the art as a "lighthouse".
The lighthouse is designed to expose the photosensitive films by projecting light from a small area radiation source through the shadow mask. Beams of light pass through the apertures of the shadow mask to form a pattern, substantially of the same shape as the apertures in the mask, on the photosensitive films. The lighthouse is designed so that these beams of light follow the same trajectories through the shadow mask as will the electron beams in the finished product. Since the color cathode-ray tube uses 3 electron guns (one for each of the three colors red, green and blue), the lighthouse will generally have some provision for adjusting the position of the light source and adjusting the optics of the lighthouse to mimic the effect of the three different electron guns. When the lighthouse is adjusted to its "red" position, the light beams projected through the shadow mask will fall on that part of the phosphor screen upon which the electron beam from the red electron gun will fall in the finished product. Similarly, when the lighthouse is adjusted to its "blue" and "green" positions, the light beams will fall on those parts of the phosphor screen upon which electrons from the blue and green guns will fall. Hence, in a succesion of photolithographic steps, the lighthouse can be used to selectively deposit red, green, and blue phosphors at the positions on the phosphor screen upon which electrons from the red, green, and blue electron guns will fall, respectively. The lighthouse may also be used to deposit a black matrix in the regions where the electron beams will not fall.
In each of these photolithographic steps, the shadow mask must be installed in its panel for the lighthouse exposure and then removed temporarily for chemical processing of the photoresist. This handling often results in dents which destroy the correspondence between the mask and the phosphor or matrix patterns laid down in previous steps. With the correspondence destroyed, the electron beams in the final product will no longer land on the intended phosphor sites. For example, electrons from the blue gun might land on red phosphors. In such a case, the tube will not reproduce colors correctly. In general, any distortion of or shift relative to the panel of the shadow mask will lead to similar problems. In addition, the faceplate panel itself may have screening defects therein, such as the contamination of one color phosphor with another, incomplete filling by phosphor or matrix, and scratches on the screen. At the present time, many of the aforementioned defects only become apparent after the electron gun assembly, funnel section and panel assembly are sealed together, and the cathode-ray tube is tested for color purity. The present invention provides a novel method of effectively testing the panel assembly for the aforementioned defects prior to the final step of frit sealing the faceplate panel to the funnel section of the tube.